Retortable plastic container having improved base stability

ABSTRACT

A retortable plastic container includes a main body and a bottom. The bottom defines a raised inner portion and at least one substantially flat bottom support surface. The substantially flat bottom support surface is curved and positioned near a radially outermost edge of the bottom when viewed in bottom plan. A groove is defined in the substantially flat bottom support surface. In addition, a first side wall portion that extends upwardly from the radially outermost edge of the bottom is shaped as a truncated cone, giving the bottom portion greater dimensional stability under retort conditions. The container bottom exhibits superior dimensional stability with respect to predecessor designs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of packaging, and morespecifically to the field of retortable plastic containers. Morespecifically, the invention relates to an improved retortable containerthat is more dimensionally stable during the sterilization process thanwere conventional predecessor containers.

2. Description of the Related Technology

Certain products require sterilization during the packaging process inorder to inhibit the growth of bacteria. Products requiringsterilization include foods such as milk, yogurt and various sauces andprepared foods, as well as certain pharmaceutical products. Thermalprocessing, sterilization, canning and retorting are all terms referringto the process of taking a food product, already sealed in itscontainer, and heating it to a specific temperature for a specific time.The objective is to kill spoilage organisms and pathogenic bacteria,thus preserving the food and allowing it to be stored unrefrigerated forextended lengths of time.

There are multiple designs for retorting food containers, includingbatch systems and continuous systems. In a batch system, containers areplaced in crates or baskets, which are then loaded into a vessel intowhich the heating medium is introduced. This method is the oldest andmost traditional and also the most versatile in the range of productsand container sizes it can handle. In a continuous retort system, aconveyor is used to continuously transport the containers to besterilized through a heating chamber that contains the heating medium.There are advantages to each method depending on individual processingoperations and, just as important, the type of food being processed.

Traditionally, products that require heat sterilization have beenpackaged in glass containers, which are relatively stable at elevatedtemperatures and pressures. However, in recent years plastic retortablecontainers have come into use. Plastic containers tend to be lessexpensive than glass containers and safer in many respects because theywill not shatter when dropped. Unfortunately, plastic containers maylack the column strength that is necessary to avoid deformation of thesidewall of the container when a number of containers or palettes ofcontainers are stacked during transportation or in packaging or retailfacilities. While it is possible to increase the strength of a plasticcontainer by increasing the thickness of the sidewall, doing so alsoincreases manufacturing costs by increasing the amount of plasticmaterial that is required. Lightweighting is an important considerationin the design of plastic containers, including plastic cans, becauseplastic material tends to be relatively expensive.

Many plastic containers also lack the requisite circumferential or hoopstrength that is required to avoid excessive deformation when thecontents of the container becomes pressurized, such as during a heatsterilization process.

The most common commercial procedure for heat sterilizing canned foodsis a retort process in which filled but unsterilized sealed cans areplaced in a retort chamber that is injected with steam and held at apredetermined elevated temperature (typically between about 210° F. toabout 260° F.) for a predetermined period of time. Conventional plasticcontainers have been considered unsuitable for packaging applications inwhich heat sterilization is required, because the heat andpressurization that is inherent to such processes has the tendency tocause irreversible damage and deformation to the sidewall of the plasticcan.

The temperatures of the retort process are elevated enough totemporarily increase the internal pressurization of the container.Plastic retortable containers accordingly have been designed to permitlimited and reversible controlled flexure of one or more surfaces inorder to accommodate the internal volumetric changes that are inherentto the retort sterilization process. U.S. Pat. No. 5,217,737 to Gygax etal. discloses a retortable plastic container that has a flexible bottomportion to accommodate internal volumetric changes. Other retortablecontainers that have been in commercial use have a champagne stylebottom portion that is designed to permit a certain amount of flexure.However, when using a continuous retort process the flexure ofretortable plastic containers must be limited so that it will notinterfere with the process of conveying the container through thecontinuous retort system. Typically, such conveyors require at least twodimensionally stable points of contact on the container.

Is also important that the bottom portions of such containers retaintheir dimensional stability after heat sterilization so that they willnot rock when placed on a horizontal support surface such as a table orretail shelf. Containers that are manufactured using an extrusion blowmolding process typically have a mold parting line or seam. In order toprevent the mold parting seam from affecting the stability of thecontainer when it is resting on a horizontal surface, recesses ortunnels are commonly integrated into the container bottom design in thearea about the parting seam. In addition, differential cooling thatoccurs during molding process can result in warping of the containerbottom that can result in rocking when the container is placed on ahorizontal surface. Such rocking can be exacerbated by additionalplastic deformation that occurs during the heat sterilization process.In the past, some designs have compensated for this warping ordistortion by molding the container bottom to have a slight undulation.However, the presence of the undulation sometimes acted as an initiationlocus or migration point for failure of the container during the heatsterilization process.

A need exists for an improved retortable container that exhibitsimproved dimensional stability and strength during the heatsterilization process without significantly adding to material costs.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a retortablecontainer that exhibits improved dimensional strength and stabilityduring the heat sterilization process without significantly adding tomaterial costs.

In order to achieve the above and other objects of the invention, aretortable plastic container according to a first aspect of theinvention includes a main body and a bottom. The bottom defines a raisedinner portion and at least one substantially flat bottom supportsurface. The substantially flat bottom support surface is curved andpositioned near a radially outermost edge of the bottom when viewed inbottom plan. A groove is defined in the substantially flat bottomsupport surface.

These and various other advantages and features of novelty thatcharacterize the invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and the objects obtainedby its use, reference should be made to the drawings which form afurther part hereof, and to the accompanying descriptive matter, inwhich there is illustrated and described a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a retortable plastic container that isconstructed according to a preferred embodiment of the invention;

FIG. 2 is a bottom plan view of the retortable plastic container that isdepicted in FIG. 1;

FIG. 3 is a cross-sectional view taken along lines 3-3 in FIG. 2; and

FIG. 4 is an enlarged portion of the area 4-4 that is depicted in FIG.3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to the drawings, wherein like reference numerals designatecorresponding structure throughout the views, and referring inparticular to FIG. 1, a retortable plastic container 10 that isconstructed according to a preferred embodiment of the inventionincludes a main body 12 and a bottom 14 that is unitary with the mainbody 12. The bottom 14 is constructed and arranged so as to be stablewhen placed on an underlying horizontal surface such as a tabletop orretail shelf.

Referring again to FIG. 1, the retortable plastic container 10 includesan upper rim 16 defining an opening 18. The main body 12 and the bottom14 are preferably fabricated from a single unitary sidewall 17 having anouter surface 19 and an inner surface 21 that defines an interior spacewithin the container that is accessible through the opening 18. Afterthe retortable plastic container 10 has been filled, a lid 31 is mountedto the rim 16 in order to seal the plastic container 10 prior to theheat sterilization process. Lid 31 is diagrammatically shown in FIG. 3.

The retortable plastic container 10 is preferably fabricated using anextrusion blow molding process from a plastic material that iscompatible with the heat sterilization process, most preferablypolypropylene. Most preferably, a multilayer material containingpolypropylene and additional layers, such as a barrier layer with lowoxygen permeability, may be used to form the sidewall 17 of the plasticcontainer 10.

Referring now to FIGS. 2-4, the container bottom 14 is preferably shapedto define a raised inner portion 20 and at least one substantially flatbottom support surface 22 that is curved when viewed in bottom plan, asis shown in FIG. 2. The substantially flat bottom support surface 22 ispreferably positioned near a radially outermost edge 23 of the bottom 14and further preferably has a groove 24 defined therein.

Groove 24 is preferably shaped so as to be substantially concentric withrespect to the curvature of the substantially flat bottom supportsurface 22, as is best shown in FIG. 2. In the preferred embodiment, thecontainer 10 has a mold parting seam 26 as a result of the extrusionblow molding process, and a pair of opposed tunnel recesses 28, 30 aredefined in the substantially flat bottom support surface 22 in the areaof mold parting seam 26. Otherwise, the substantially flat bottomsupport surface 22 has a substantially annular shape defined by asubstantially constant inner radius and a substantially constant outerradius, and preferably extends continuously around the outer peripheryof the container bottom 14.

As FIG. 4 best shows, the substantially flat bottom support surface 22has a width W_(S), which is defined as the radial distance of thelowermost surface of the support surface 22. The surface preferablyresides substantially within a plane that will contact an underlyinghorizontal surface on which the container 10 is resting. The container10 has a maximum outer diameter D_(MAX), as is depicted in FIG. 3.Preferably, a ratio W_(S)/D_(MAX) of the width W_(S) to the maximumouter diameter D_(MAX) is substantially within a range of about 0.035 toabout 0.2, more preferably substantially within a range of about 0.0425to about 0.01 and most preferably substantially within a range of about0.05 to about 0.008.

The groove 24 preferably has a maximum width W_(G), which is best shownin FIG. 4. Preferably, a ratio W_(G)/W_(S) of the maximum width W_(G) ofthe groove to the width W_(S) of the support surface is substantiallywithin a range of about 0.1 to about 0.9, more preferably substantiallywithin a range of about 0.25 to about 0.75 and most preferablysubstantially within a range of about 0.40 to about 0.60.

The groove 24 preferably has a curved inner surface 32 that has anaverage radius of curvature R_(G), as is best shown in FIG. 4.Preferably, a ratio R_(G)/W_(S) of the average radius of curvature R_(G)to the width W_(S) of the support surface is substantially within arange of about 0.05 to about 1.0, more preferably substantially within arange of about 0.25 to about 0.75 and most preferably substantiallywithin a range of about 0.40 to about 0.60.

The groove 24 also preferably has a maximum depth D_(G), also best shownin FIG. 4. Preferably, a ratio D_(G)/W_(S) of the maximum depth of thegroup 24 to the width W_(S) of the support surface is substantiallywithin a range of about 0.01 to about 0.30, more preferablysubstantially within a range of as 0.03 to about 0.20 and mostpreferably substantially within a range of about 0.05 to about 0.15.

As FIG. 4 also best shows, the outermost edge 23 of the of supportsurface 22 is preferably convex and has an average radius of curvatureR_(O). Preferably, a ratio R_(O)/D_(G) of the average radius ofcurvature R_(O) of the outermost edge 23 to the maximum depth D_(G) ofthe groove 24 is preferably within a range of about 0.5 to about 10.0,more preferably substantially within a range of about 0.75 to about 5.0and most preferably substantially within a range of about 1.0 to about3.0.

As FIG. 4 also shows, the sidewall 17 in the area of the bottom 14preferably has an average thickness W_(T). Preferably, a ratio of theaverage thickness W_(T) to the average radius of curvature R_(G) issubstantially within a range of about 0.5 to about 2.0, more preferablysubstantially within a range of about 0.7 to about 1.8 and mostpreferably substantially within a range of about 0.9 to about 1.6.

Preferably, a ratio W_(G)/W_(T) of the maximum width of the groove 24 tothe average thickness of the side wall 17 is substantially within arange of about 0.1 to about 1.0, more preferably substantially withinrange of about 0.2 to about 0.85 and most preferably substantially therange about 0.3 to about 0.65.

Referring again the FIG. 4, it will be seen that the sidewall 17 abovethe outermost edge 23 of the support surface 22 has a first sidewallportion 40 and a second sidewall portion 42. The first sidewall portion40 is preferably substantially straight when viewed in side elevationand extends circumferentially about the outer periphery of the lower endof the container 10 directly above the outermost edge 23 to form atruncated inverted cone shape. As viewed in longitudinal cross-sectionas shown in FIG. 4, the first sidewall portion defines an angle a withrespect to a vertical axis that preferably substantially within a rangeof about zero degrees to about 15 degrees, more preferably substantiallywithin a range of about zero degrees to about 12 degrees and mostpreferably substantially within a range of about zero degrees to about 8degrees.

The first sidewall portion further has a height H₁. Preferably, a ratioH₁/D_(MAX) of the height H₁ of the first sidewall portion to the maximumouter diameter D_(MAX) of the container 10 is substantially within arange of about 0.015 to about 0.040, more preferably substantiallywithin a range of about 0.020 to about 0.035 and most preferablysubstantially within a range of about 0.025 to about 0.030.

The second sidewall portion 42 is unitary with the first sidewallportion 40 and a connected thereto by a concave fillet having an averageradius of curvature R_(B). Preferably, a ratio R_(B)/D_(MAX) of theaverage radius of curvature R_(B) to the maximum outer diameter D_(MAX)of the container 10 is substantially within a range of about 0.0045 toabout 0.065, more preferably substantially within a range of about 0.007to about 0.04 and most preferably substantially within a range of about0.0092 to about 0.02.

The presence of the groove 24 and the division of the support surface 22into inner and outer portions in conjunction with the truncated coneshape of the first sidewall portion 40 has the effect of stiffening thebottom 14 during the heat sterilization process, and also has thetendency to preserve the flatness of the support surface 22 both duringthe heat sterilization process and during subsequent use of thecontainer 10. The stiffening effect is particularly pronounced whenusing polypropylene.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

What is claimed is:
 1. A retortable plastic container, comprising: amain body; and a bottom; the bottom defining a raised inner portion andat least one substantially flat bottom support surface that is curvedand positioned near a radially outermost edge of the bottom when viewedin bottom plan, and further comprising a groove defined in thesubstantially flat bottom support surface.
 2. A retortable plasticcontainer according to claim 1, wherein the groove is substantiallyconcentric with respect to the curvature of the substantially flatbottom support surface.
 3. A retortable plastic container according toclaim 1, wherein the bottom is fabricated from a material comprisingpolypropylene.
 4. A retortable plastic container according to claim 1,wherein the retortable plastic container is fabricated using anextrusion blow molding process.
 5. A retortable plastic containeraccording to claim 4, wherein the plastic container has a mold partingline as a result of the extrusion blow molding process, wherein thesubstantially flat bottom support surface extends continuously around aperiphery of the bottom, and wherein a pair of opposed tunnel recessesare defined in the substantially flat bottom support surface in the areaof the mold parting line.
 6. A retortable plastic container according toclaim 1, wherein the substantially flat bottom support surface has awidth, and wherein the retortable plastic container has a maximum outerdiameter, and wherein a ratio of the width to the maximum outer diameteris substantially within a range of about 0.035 to about 0.2.
 7. Aretortable plastic container according to claim 6, wherein the ratio ofthe width to the maximum outer diameter is substantially within a rangeof about 0.0425 to about 0.01.
 8. A retortable plastic containeraccording to claim 7, wherein a ratio of the width to the maximum outerdiameter is substantially within a range of about 0.05 to about 0.008.9. A retortable plastic container according to claim 1, wherein thegroove has a curved inner surface having a maximum width, and whereinthe substantially flat bottom support surface has a width, and wherein aratio of the maximum width of the groove to the width of the supportsurface is substantially within a range of about 0.1 to about 0.9.
 10. Aretortable plastic container according to claim 9, wherein the ratio ofthe maximum width of the groove to the width of the support surface issubstantially within a range of about 0.25 to about 0.75.
 11. Aretortable plastic container according to claim 10, wherein the ratio ofthe maximum width of the groove to the width of the support surface issubstantially within a range of about 0.40 to about 0.6.
 12. Aretortable plastic container according to claim 1, wherein the groovehas a curved inner surface having an average radius of curvature andwherein the substantially flat bottom support surface has a width, andwherein a ratio of the average radius of curvature to the width of thesupport surface is substantially within a range of about 0.05 to about1.0.
 13. A retortable plastic container according to claim 12, whereinthe ratio of the average radius of curvature to the width of the supportsurface is substantially within a range of about 0.25 to about 0.75. 14.A retortable plastic container according to claim 13, wherein the ratioof the average radius of curvature to the width of the support surfaceis substantially within a range of about 0.40 to about 0.6.
 15. Aretortable plastic container according to claim 1, wherein the groovehas a maximum depth and wherein the support surface has a width, andwherein a ratio of the maximum depth of the groove to the width of thesupport surface is substantially within a range of about 0.01 to about0.3.
 16. A retortable plastic container according to claim 15, whereinthe ratio of the maximum depth of the groove to the width the supportsurface is substantially within a range of about 0.03 to about 0.2. 17.A retortable plastic container according to claim 16, wherein the ratioof the maximum depth of the groove to the width of the support surfaceis substantially within a range of about 0.05 to about 0.15.
 18. Aretortable plastic container according to claim 1, wherein the groovehas a maximum depth and wherein an outermost edge of the support surfaceis convex and has an average radius of curvature, and wherein a ratio ofthe average radius of curvature of the outermost edge to the maximumdepth of the groove is substantially within a range of about 0.5 toabout
 10. 19. A retortable plastic container according to claim 18,wherein the ratio of the average radius of curvature of the outermostedge to the maximum depth of the groove is substantially within a rangeof about 0.75 to about
 5. 20. A retortable plastic container accordingto claim 19, wherein the ratio of the average radius of curvature of theoutermost edge to the maximum depth of the groove is substantiallywithin a range of about 1.0 to about 3.0.
 21. A retortable plasticcontainer according to claim 1, wherein the groove has a curved innersurface having an average radius of curvature and wherein the sidewallhas an average thickness, and wherein a ratio of the average thicknessto the average radius of curvature is substantially within a range ofabout 0.5 to about 2.0.
 22. A retortable plastic container according toclaim 21, wherein the ratio of the average thickness to the averageradius of curvature is substantially within a range of about 0.7 toabout 1.8.
 23. A retortable plastic container according to claim 22,wherein the ratio of the average thickness to the average radius ofcurvature is substantially within a range of about 0.9 to about 1.6. 24.A retortable plastic container according to claim 1, wherein the groovehas a maximum width and wherein the sidewall has an average thickness,and wherein a ratio of the average thickness to the maximum width issubstantially within a range of about 0.1 to about 1.0.
 25. A retortableplastic container according to claim 24, wherein the ratio of theaverage thickness to the maximum width is substantially within a rangeof about 0.2 to about 0.85.
 26. A retortable plastic container accordingto claim 25, wherein the ratio of the average thickness to the maximumwidth is substantially with a range of about 0.3 to about 0.65.
 27. Aretortable plastic container according to claim 1, wherein the containerfurther comprises a first side wall portion extending upwardly from theradially outermost edge, and wherein the first side wall portion issubstantially straight when viewed in longitudinal cross-section.
 28. Aretortable plastic container according to claim 27, wherein the firstside wall portion defines an angle with respect to a vertical axis thatis substantially within a range of about 0° to about 15°.
 29. Aretortable plastic container according to claim 28, wherein the firstside wall portion defines an angle with respect to a vertical axis thatis substantially within a range of about 0° to about 12°.
 30. Aretortable plastic container according to claim 29, wherein the firstside wall portion defines an angle with respect to a vertical axis thatis substantially within a range of about 0° to about 8°.
 31. Aretortable plastic container according to claim 27, wherein the firstside wall portion is shaped substantially as a truncated cone.
 32. Aretortable plastic container according to claim 27, further comprising asecond side wall portion extending upwardly from the first side wallportion, and wherein the second side wall portion is connected to thefirst side wall portion by a concave fillet having an average radius ofcurvature.
 33. A retortable plastic container according to claim 27,wherein the first side wall portion has a maximum height, and wherein aratio of the maximum height to a maximum outer diameter of the containeris substantially within a range of about 0.015 to about 0.040.
 34. Aretortable plastic container according to claim 33, wherein the ratio ofthe maximum height to the maximum outer diameter of the container issubstantially within a range of about 0.020 to about 0.035.
 35. Aretortable plastic container according to claim 34, wherein the ratio ofthe maximum height to the maximum outer diameter of the container issubstantially within a range of about 0.025 to about 0.030.
 36. Aretortable plastic container according to claim 27, further comprising asecond sidewall portion that is connected to the first sidewall portionby a fillet having average radius of curvature, and wherein a ratio ofthe average radius of curvature to the maximum outer diameter of thecontainer is substantially within a range of about 0.0045 to about0.065.
 37. A retortable plastic container according to claim 36, whereinthe ratio of the average radius of curvature to the maximum outerdiameter of the container is substantially within a range of about 0.007to about 0.04.
 38. A retortable plastic container according to claim 37,wherein the ratio of the average radius of curvature to the maximumouter diameter of the container is substantially within a range of about0.0092 to about 0.02.